crypto: aesni - Fix out-of-bounds access of the AAD buffer in generic-gcm-aesni

The aesni_gcm_enc/dec functions can access memory after the end of
the AAD buffer if the AAD length is not a multiple of 4 bytes.
It didn't matter with rfc4106-gcm-aesni as in that case the AAD was
always followed by the 8 byte IV, but that is no longer the case with
generic-gcm-aesni. This can potentially result in accessing a page that
is not mapped and thus causing the machine to crash. This patch fixes
that by reading the last <16 byte block of the AAD byte-by-byte and
optionally via an 8-byte load if the block was at least 8 bytes.

Fixes: 0487ccac ("crypto: aesni - make non-AVX AES-GCM work with any aadlen")
Cc: <stable@vger.kernel.org>
Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>